The present invention relates to a magnetic random access memory (MRAM) and a fabricating method thereof, and more particularly to a method of forming an MRAM cell having a bottom sense layer.
Magnetic random access memories (MRAMs) employ memory cells having magnetic multilayer films as storage elements. When in use, an MRAM cell stores information as digital bits, in the form of relative magnetic orientations of spaced thin magnetic multilayer films forming each memory cell. Each MRAM cell has two stable magnetic film orientations, one which produces a high resistance across the cell representing e.g. a logic state 0 and another which produces a lower resistance across the cell representing e.g. a logic state 1, or vice versa.
A typical multilayer-film MRAM array includes a number of bit or digit (column) lines intersected by a number of word (row) lines. An MRAM cell is formed between a digit and row line at each intersection.
The basic memory MRAM cell has a first pinned ferromagnetic layer and a second free (sense) magnetic layer with a nonmagnetic layer between them. The pinned ferromagnetic layer has a fixed magnetic orientation while the free (sense) layer may have two different magnetic orientations, depending on the logical data stored in the cell. Each of the ferromagnetic layers is actually formed of a stack of as many as ten different overlapping material layers. Fabrication of such stacks requires deposition of the thin materials layer by layer, according to a predefined order.
FIG. 1 shows an exemplary conventional MRAM structure including MRAM cells 22 formed as layer stacks which have three respective associated bit or digit lines 18. The digit lines 18, typically formed of copper (Cu), are first formed in an insulating layer 16 formed over underlayers 14 of an integrated circuit (IC) substrate 10. Underlayers 14 may include, for example, portions of integrated circuitry, such as CMOS circuitry. A pinned layer 20, typically formed of ferromagnetic materials, is provided over each digit line 18. A nonmagnetic layer of, for example, Al2O3 is formed over the pinned layer 20. A free (sense) layer 21 is provided over the nonmagnetic layer. The MRAM cells 22 are coupled to a word line 23 that intersects three pinned layers 20 and associated cells 22. The word line 23 and bit line 18 may also be interchanged.
FIG. 2 illustrates a side sectional view of the MRAM stacks 22 of FIG. 1. As shown, pinned layer 20 and sense layer 21 are comprised of several individual layers, including a bottom conductive barrier layer 24 formed of, for example, Ta, at the base of the pinned layer 20. The barrier layer 20 also lines the trenches in which the bit lines 18 are formed. Also, pinned layer 20 and sense layer 21 are separated by a magnetically and electrically nonconductive tunnel junction layer 25, for example, A12O3.
Typically, during an etching step to define the cells 22, utilizing, for example, ion milling, the conductive layer 24 may sputter back onto the sidewall of stacks 22 forming a side conductive layer 26 creating an undesirable electrical short between the pinned 20 and sense 21 layers. Thus, during a read operation, the current may flow through the side conductive layer 26 rather than flow through the tunnel junction layer 25, because of a short produced by layer 26 causing improper resistance sensing. Hence, what is needed is a method of fabricating an MRAM cell which will not create a short as described above.
The present invention provides a method of forming an MRAM bit having a bottom sense layer utilizing electroless plating. In an exemplary embodiment of the invention, a first conductor in a trench is provided in an insulating layer and an upper surface of the insulating layer and the first conductor is planarized. Then, a dielectric layer is deposited to a thickness slightly greater than the desired thickness of the sense layer, which is formed later. The dielectric layer is then patterned and etched to form an opening or formed cell shapes over the first conductor. Then, a permalloy is electroplated in the formed cell shapes to form the sense layer. The formed sense layer is pre-sputter cleaned and then a tunnel barrier layer is deposited. Finally, the pinned layer is formed over the tunnel barrier layer.
The above advantages and features of the invention as well as others will be more clearly understood from the following detailed description which is provided in connection with the accompanying drawings.